Non-oriented electrical steel sheet and its manufacturing method

Abstract

Provided is a non-oriented electrical steel sheet that is excellent in iron loss and magnetic anisotropy under high magnetic flux and high frequency conditions, and a method for manufacturing the same. [Solution] A method for producing a grain-oriented electrical steel sheet includes the steps of heating a slab containing, by weight, 1.5 to 6.5% Si, 0.0005 to 3.5% Al, 0.01 to 3.0% Mn, 0.005 to 5.0% Cr, 0.0005 to 0.03% S, the balance being Fe and other unavoidable impurities, at 1050 to 1220°C, finish hot rolling the slab to obtain a hot-rolled sheet, hot-rolling the hot-rolled sheet at 850 to 1150°C for 30 to 300 seconds, cold-rolling the hot-rolled sheet that has been annealed to obtain a cold-rolled sheet, heating the cold-rolled sheet, and rolling the heated cold-rolled sheet. and final annealing at 600 to 1150°C for 10 to 300 seconds, wherein the cold rolling satisfies the following [Relationship 2], wherein the heating rate in the temperature range of 300 to 500°C during heating is 5 to 150°C / s, and the gas atmosphere in the temperature range of 500 to 750°C during the final annealing is composed of, by volume, 15 to 99.99% hydrogen, 0.0001 to 0.0030% oxygen, and the balance being an inert gas, and the oxygen content in the gas atmosphere in the temperature range of 800 to 1100°C during the final annealing is 1 to 1.5 times that in the temperature range of 500 to 750°C. [Relationship 2] Maximum temperature of cold-rolled sheet surface during cold rolling < 200 × cold rolling reduction / 100 + 40

Description

[Technical Field]

[0001] The present invention relates to a non-oriented electrical steel sheet and a method for manufacturing the same. [Background technology]

[0002] Electric vehicles and other forms of transportation are being used to replace internal combustion engines in order to reduce greenhouse gas emissions. Currently, more than 50% of all electrical energy generated is consumed by electric motors for power generation. Therefore, efficient use of electricity is crucial for the future replacement of internal combustion engines with electric motors. In particular, electric motors are becoming increasingly compact and lightweight, along with their improved performance. Motors that generate axial magnetic flux are gaining attention. These motors eliminate spatial constraints and improve performance in a variety of electric mechanisms, including existing electric vehicles, electric motorcycles, electric aircraft, and electric ships. Therefore, high efficiency is essential. Furthermore, technological advances and market changes are driving continued interest in improving the functionality and efficiency of high-efficiency motors for home appliances, robots, and industrial motors, leading to ever-increasing demand for more efficient use of electrical energy.

[0003] Meanwhile, optimizing all aspects of electric motor efficiency, from material selection to design, assembly, and control, is crucial. In terms of materials, the magnetic properties of electrical steel sheets are particularly important, with high demands for low iron loss and high magnetic flux density. For automobile drive motors and air conditioner compressor motors, which must operate not only in the commercial frequency range but also in the high-frequency range, low iron loss at high frequencies is crucial. Furthermore, for small, high-power motors with narrow yokes, high magnetic flux is generated not only in the motor's teeth but also in the yoke to generate high torque during motor operation. Therefore, improving iron loss at high frequencies and high magnetic flux is crucial to improving motor efficiency.

[0004] Typically, electrical steel sheets are manufactured by adding large amounts of resistive elements such as Si, Al, and Mn to reduce the grain size and eddy current loss. As the frequency increases, eddy currents pass only through the surface layer of the steel sheet, so high-frequency iron loss can be improved by increasing the resistivity of the elements in the surface layer. However, while this conventional manufacturing method is effective in controlling iron loss at magnetic fluxes around 1.0 T, no method is known for reducing iron loss at higher magnetic fluxes. Summary of the Invention [Problem to be solved by the invention]

[0005] The present invention provides a non-oriented electrical steel sheet that is excellent in core loss and magnetic anisotropy under high magnetic flux and high frequency conditions, and a method for manufacturing the same. [Means for solving the problem]

[0006] The non-oriented electrical steel sheet of the present invention is characterized in that it contains, by weight %, 1.5 to 6.5% Si, 0.0005 to 3.5% Al, 0.01 to 3.0% Mn, 0.005 to 5.0% Cr, 0.0005 to 0.03% S, with the balance being Fe and other unavoidable impurities, and that the Goss orientation fraction in the region from the surface to 1 / 10t in the thickness direction (t: steel sheet thickness) is 3 area % or less, and the density of inclusions in the region from the surface to 1 / 50t in the thickness direction (t: steel sheet thickness) is the same as or less than the density of inclusions in other regions.

[0007] The non-oriented electrical steel sheet may further contain one or more of P: 0.005 to 0.08% and Sn: 0.01 to 0.2%.

[0008] The non-oriented electrical steel sheet may further contain one or more of C: 0.005% or less (excluding 0%), N: 0.005% or less (excluding 0%), O: 0.005% or less (excluding 0%), and Ti: 0.01% or less (excluding 0%).

[0009] The non-oriented electrical steel sheet may further contain one or more of Mo: 0.1% or less (excluding 0%), B: 0.0050% or less (excluding 0%), V: 0.050% or less (excluding 0%), Ca: 0.010% or less (excluding 0%), Nb: 0.0050% or less (excluding 0%), and Mg: 0.0050% or less (excluding 0%).

[0010] The non-oriented electrical steel sheet may further contain one or more of Sb: 0.1% or less (excluding 0%), Ni: 0.05% or less (excluding 0%), Cu: 0.005 to 0.2%, and Zn: 0.01% or less (excluding 0%).

[0011] The non-oriented electrical steel sheet may further contain one or more of Bi, Pb, Ge and As in an amount of 0.20% or less (excluding 0%), either individually or in total.

[0012] The non-oriented electrical steel sheet can satisfy the following relational expression 1.

[0013] [Equation 1] Si>Al+Mn

[0014] The non-oriented electrical steel sheet may have a Goss orientation fraction of 5% by area or less across the entire thickness direction.

[0015] The non-oriented electrical steel sheet may have an iron loss (W15 / 1000L) of 150 W / Kg or less and an iron loss (W15 / 1000C) of 150 W / Kg or less.

[0016] The non-oriented electrical steel sheet may satisfy the condition 2×(W15 / 1000C−W15 / 1000L) / (W15 / 1000C+W15 / 1000L)<0.1.

[0017] Further, a method for producing a non-oriented electrical steel sheet of the present invention includes the steps of heating a slab containing, by weight, 1.5 to 6.5% Si, 0.0005 to 3.5% Al, 0.01 to 3.0% Mn, 0.005 to 5.0% Cr, 0.0005 to 0.03% S, with the balance being Fe and other inevitable impurities, at 1050 to 1220°C; finish hot rolling the slab to obtain a hot-rolled sheet; hot-annealing the hot-rolled sheet at 850 to 1150°C for 30 to 300 seconds; cold-rolling the hot-annealed hot-rolled sheet to obtain a cold-rolled sheet; heating the cold-rolled sheet; and rolling the heated cold-rolled sheet. and final annealing the sheet at 600 to 1150°C for 10 to 300 seconds, wherein the cold rolling satisfies the following [Relationship 2], the heating rate in the temperature range of 300 to 500°C during the heating is 5 to 150°C / s, the gas atmosphere in the temperature range of 500 to 700°C during the final annealing is composed of, by volume, 15 to 99.99% hydrogen, 0.0001 to 0.0030% oxygen, and the remainder an inert gas, and the oxygen content in the gas atmosphere in the temperature range of 800 to 1100°C during the final annealing is 1 to 1.5 times that in the temperature range of 500 to 750°C.

[0018] [Equation 2] Maximum temperature on the surface of cold-rolled sheet during cold rolling <200 × cold rolling reduction / 100 + 40

[0019] The slab may further contain one or more of P: 0.005 to 0.08% and Sn: 0.01 to 0.2%.

[0020] The slab may further contain one or more of C: 0.005% or less (excluding 0%), N: 0.005% or less (excluding 0%), O: 0.005% or less (excluding 0%), and Ti: 0.01% or less (excluding 0%).

[0021] The slab may further contain one or more of Mo: 0.1% or less (excluding 0%), B: 0.0050% or less (excluding 0%), V: 0.050% or less (excluding 0%), Ca: 0.010% or less (excluding 0%), Nb: 0.0050% or less (excluding 0%), and Mg: 0.0050% or less (excluding 0%).

[0022] The slab may further contain one or more of Sb: 0.1% or less (excluding 0%), Ni: 0.05% or less (excluding 0%), Cu: 0.005 to 0.2%, and Zn: 0.01% or less (excluding 0%).

[0023] The slab may further contain one or more of Bi, Pb, Ge and As in an amount of 0.20% or less (excluding 0%), either individually or in total.

[0024] The slab can satisfy the following relational expression 1.

[0025] [Equation 1] Si>Al+Mn

[0026] The finish hot rolling can be carried out at 700 to 1050°C.

[0027] The cold rolling may be performed at a cold reduction rate of 35 to 98%.

[0028] During the cold rolling, the maximum rolling speed in one or more of the first and second passes may be 3 m / s or more. [Effects of the Invention]

[0029] According to the present invention, it is possible to provide a non-oriented electrical steel sheet that is excellent in core loss and magnetic anisotropy under high magnetic flux and high frequency conditions, and a method for manufacturing the same. DETAILED DESCRIPTION OF THE INVENTION

[0030] The non-oriented electrical steel sheet of the present invention will be described below. First, the alloy composition will be described. The contents of the alloy compositions described below are in weight percent unless otherwise specified.

[0031] Si: 1.5 to 6.5% Si plays a role in increasing the resistivity of the material and reducing iron loss. If the Si content is less than 1.5%, the effect of improving high-frequency iron loss is insufficient. If the Si content exceeds 6.5%, hardness increases, and productivity and punchability deteriorate. Therefore, the Si content is preferably in the range of 1.5 to 6.5%. The lower limit of the Si content is more preferably 1.8%, and even more preferably 2.0%. The upper limit of the Si content is more preferably 6.0%, even more preferably 5.0%, and most preferably 4.0%.

[0032] Al: 0.0005 to 3.5% Al increases the resistivity of the material and reduces iron loss. If the Al content is less than 0.0005%, the amount of Al required to remove oxygen during steel production is reduced, resulting in the formation of many inclusions in the steel, which may result in an ineffective reduction of high-frequency iron loss and the formation of fine nitrides on the surface, potentially reducing magnetic properties. If the Al content exceeds 3.5%, problems may arise in all processes, including steelmaking and continuous casting, significantly reducing productivity. Therefore, the Al content is preferably in the range of 0.0005 to 3.5%. The lower limit of the Al content is more preferably 0.15%. The upper limit of the Al content is more preferably 3.0%, even more preferably 2.5%, and most preferably 2.0%.

[0033] Mn: 0.01 to 3.0% Mn increases the resistivity of the material, improving iron loss, and plays a role in forming sulfides, stabilizing austenite. If the Mn content is less than 0.01%, fine sulfides, such as MnS, may precipitate in the steel, reducing magnetic properties. If the Mn content exceeds 3.0%, the annealing temperature range for obtaining an appropriate grain size for low high-frequency iron loss is limited. Furthermore, Mn may reduce the saturation magnetic flux of the material, particularly promoting the formation of a {111} texture that is unfavorable to ferromagnetism, resulting in a decrease in magnetic flux density. Therefore, the Mn content is preferably in the range of 0.01 to 3.0%. The lower limit of the Mn content is more preferably 0.2%. The upper limit of the Mn content is more preferably 2.5%, even more preferably 2.0%, and most preferably 1.5%.

[0034] Cr: 0.005 to 5.0% Cr increases the resistivity of the material and reduces iron loss. By appropriately controlling the cold rolling and final annealing conditions, Cr can concentrate on the surface to form a Cr-enriched layer. If the Cr content is less than 0.005%, the effect of Cr in increasing the resistivity is minimal, and Cr may combine with C and other elements to form carbides that are detrimental to magnetic properties. If the Cr content exceeds 5%, Cr is distributed uniformly throughout the entire thickness rather than on the surface, resulting in a decrease in magnetic flux density throughout the steel sheet. Therefore, the Cr content is preferably in the range of 0.005 to 5.0%. From the viewpoint of surface enrichment, the lower limit of the Cr content is more preferably 0.04%. From the viewpoint of preventing a decrease in magnetic flux density, the upper limit of the Cr content is more preferably 3.0%, even more preferably 1.0%, and most preferably 0.3%.

[0035] S: 0.0005 to 0.03% S is a strong segregating element and forms precipitates. When a titrated amount is added, it segregates to the surface and reacts with oxygen in the air during annealing, thereby suppressing the growth of Goss crystal grains on the surface. If the S content is less than 0.0005%, the surface segregation effect becomes insufficient. If the S content exceeds 0.03%, an FeS-enriched layer forms on the surface, significantly deteriorating the surface quality. Therefore, the S content is preferably in the range of 0.0005 to 0.03%. The lower limit of the S content is more preferably 0.001%. The upper limit of the S content is more preferably 0.015%, even more preferably 0.005%, and most preferably 0.0035%.

[0036] The remaining component is iron (Fe). However, in a normal manufacturing process, unintentional impurities may be inevitably mixed in from the raw materials or the surrounding environment, and this cannot be excluded. These impurities are known to anyone skilled in the normal manufacturing process, so this specification does not specifically mention all of them.

[0037] The non-oriented electrical steel sheet of the present invention may further contain one or more of P: 0.005 to 0.08% and Sn: 0.01 to 0.2%.

[0038] P: 0.005 to 0.08% P concentrates on the surface and plays a role in controlling the fraction of the internal oxide layer. If the P content is less than 0.005%, it may be difficult to form a uniform internal oxide layer. If the P content is more than 0.08%, the melting point of the Si-based oxide may fluctuate, which may lead to the rapid formation of the internal oxide layer. Therefore, the P content is preferably in the range of 0.005 to 0.08%. The upper limit of the P content is more preferably 0.07%.

[0039] Sn: 0.01 to 0.2% Sn segregates on the surface and grain boundaries of the steel sheet, suppressing surface oxidation during annealing and improving texture. If the Sn content is less than 0.01%, it may be difficult to fully achieve the above-mentioned effects. If the Sn content exceeds 0.2%, Sn segregates on the grain boundaries, reducing toughness and decreasing productivity relative to magnetic improvement. Therefore, the Sn content is preferably in the range of 0.01 to 0.2%. The lower limit of the Sn content is more preferably 0.02%. The upper limit of the Sn content is more preferably 0.15%, even more preferably 0.1%, and most preferably 0.07%.

[0040] The non-oriented electrical steel sheet of the present invention may further contain one or more of C: 0.005% or less (excluding 0%), N: 0.005% or less (excluding 0%), O: 0.005% or less (excluding 0%), and Ti: 0.01% or less (excluding 0%).

[0041] C: 0.005% or less (excluding 0%) C reacts with N, Ti, Nb, V, etc. to form fine carbides, which play a role in hindering grain growth and magnetic domain movement, so the upper limit is limited to 0.005%.

[0042] N: 0.005% or less (excluding 0%) N combines with Ti, Nb, V, etc. to form nitrides, which act to reduce grain growth, so the upper limit is set to 0.005%.

[0043] O: 0.005% or less (excluding 0%) O reacts with Fe, Ti, Al, Mn, Cr, Si, V, etc. to form fine oxides, which play a role in hindering grain growth and magnetic domain movement, so the upper limit is limited to 0.005%.

[0044] Ti: 0.01% or less (excluding 0%) Ti combines with C, N, O, etc. to form fine nitrides or oxides, which plays a role in preventing magnetic domain movement, so the upper limit is limited to 0.01%.

[0045] The non-oriented electrical steel sheet of the present invention may further contain one or more of Mo: 0.1% or less (excluding 0%), B: 0.0050% or less (excluding 0%), V: 0.050% or less (excluding 0%), Ca: 0.010% or less (excluding 0%), Nb: 0.0050% or less (excluding 0%), and Mg: 0.0050% or less (excluding 0%).

[0046] Mo: 0.1% or less (excluding 0%) Mo reacts with C, O, N, etc. to form fine carbides or nitrides, which adversely affect the magnetic properties, so the upper limit is set to 0.1%.

[0047] B: 0.0050% or less (excluding 0%) B reacts with C, O, N, etc. to form fine carbides or nitrides, which adversely affect magnetic properties, so the upper limit is set to 0.0050%.

[0048] V: 0.050% or less (excluding 0%) V reacts with C, O, N, etc. to form fine carbides or nitrides, which adversely affect magnetic properties, so the upper limit is set to 0.050%.

[0049] Ca: 0.010% or less (excluding 0%) Ca reacts with C, O, N, etc. to form fine carbides or nitrides, which have a negative effect on magnetic properties, so the upper limit is set to 0.010%.

[0050] Nb: 0.0050% or less (excluding 0%) Nb reacts with C, O, N, etc. to form fine carbides or nitrides, which adversely affect magnetic properties, so the upper limit is set to 0.0050%.

[0051] Mg: 0.0050% or less (excluding 0%) Mg reacts with C, O, N, etc. to form fine carbides or nitrides, which adversely affect magnetic properties, so the upper limit is set to 0.0050%.

[0052] The non-oriented electrical steel sheet of the present invention may further contain one or more of Sb: 0.1% or less (excluding 0%), Ni: 0.05% or less (excluding 0%), Cu: 0.005 to 0.2%, and Zn: 0.01% or less (excluding 0%).

[0053] Sb: 0.1% or less (excluding 0%) Sb is an element that segregates at grain boundaries, suppressing the diffusion of nitrogen through the grain boundaries, suppressing the {111} texture that is detrimental to magnetic properties, and increasing the advantageous {100} texture, thereby improving magnetic properties. If the Sb content exceeds 0.1%, it will impede grain growth, reducing magnetic properties and deteriorating rolling properties. More specifically, the Sb content may be 0.001 to 0.1%. Even more specifically, the Sb content may be 0.005 to 0.08%.

[0054] Ni: 0.05% or less (excluding 0%) Ni reacts with impurity elements to form fine sulfides, carbides, and nitrides, which have a detrimental effect on magnetic properties, so its upper limit is limited to 0.05%. More specifically, the Ni content may be 0.0001 to 0.050%. Even more specifically, the Ni content may be 0.001 to 0.030%.

[0055] Cu: 0.005 to 0.2% Cu plays a role in forming sulfides together with Mn. If the Cu content is less than 0.005%, fine (Cu Mn)S precipitates, which may deteriorate the magnetic properties. If the Cu content exceeds 0.2%, high-temperature embrittlement may occur, which may cause cracks during continuous casting or hot rolling. More specifically, the Cu content may be 0.010 to 0.1%.

[0056] Zn: 0.01% or less (excluding 0%) Zn acts as an impurity and may deteriorate magnetic properties, so its upper limit is set to 0.01%. More specifically, the Zn content may be 0.0001 to 0.01%. Even more specifically, the Zn content may be 0.001 to 0.008%.

[0057] The non-oriented electrical steel sheet of the present invention may further contain one or more of Bi, Pb, Ge and As in an amount of 0.20% or less (excluding 0%), either individually or in total.

[0058] When the above-mentioned elements are added, they segregate at the grain boundaries, alleviating stress concentration at the grain boundaries during cold rolling, and in the subsequent recrystallization annealing process, <111> / / Improves magnetic flux density by suppressing recrystallization of ND-oriented crystal grains. When these elements are added appropriately, the above-mentioned effects can be further achieved. However, excessive addition can cause significant segregation, suppressing grain growth and potentially deteriorating magnetic flux density and core loss. More specifically, the alloy may contain one or more of Bi, Pb, Ge, and As, each in an amount of 0.0001 to 0.20%. Even more specifically, the alloy may contain one or more of Bi, Pb, Ge, and As, each in an amount of 0.001 to 0.10%.

[0059] The non-oriented electrical steel sheet of the present invention can satisfy the following relational expression 1.

[0060] [Relationship 1] Si>Al+Mn

[0061] If the above relational expression 1 is not satisfied, the magnetic anisotropy index, which is a physical property of the steel sheet, cannot be sufficiently reduced, and there is a possibility that the magnetic anisotropy at high frequencies will increase significantly.

[0062] In the non-oriented electrical steel sheet of the present invention, the Goss orientation fraction in the region from the surface to 1 / 10t (t: steel sheet thickness) in the thickness direction is preferably 3 area % or less. The Goss orientation fraction can be the area fraction of crystal grains having an orientation within 10° from the Goss orientation. While the Goss orientation provides excellent magnetic properties in the rolling direction, it has a significant adverse effect on magnetic properties in the direction perpendicular to the rolling direction. Therefore, in order to improve magnetic properties in the direction perpendicular to the rolling direction, it is very important to reduce the fraction of crystal grains having the Goss orientation. In particular, the surface layer has a greater effect on the overall iron loss as the frequency increases. If the Goss orientation fraction in the region from the surface to 1 / 10t (t: steel sheet thickness) in the thickness direction exceeds 3 area %, it will have a significant adverse effect on magnetic properties in the direction perpendicular to the rolling direction. Meanwhile, the Goss orientation fraction measurement method involves measuring the cross section of a steel sheet using conventional EBSD, but statistical significance is achieved by ensuring that the number of crystal grains with a minimum crystal grain size of over 5 μm, which have grain boundaries with a misorientation angle of 3° or more with surrounding crystal grains, is at least 5,000 within the measurement area. For crystal grains measured in a cut-out form within the measurement area, the cut-out area is included in the calculation of the fraction.

[0063] The non-oriented electrical steel sheet of the present invention may have a Goss orientation fraction of 5% by area or less across the entire region in the thickness direction. If the Goss orientation fraction across the entire region in the thickness direction exceeds 5% by area, this may adversely affect the magnetic properties in the direction perpendicular to the rolling direction.

[0064] In the non-oriented electrical steel sheet of the present invention, the density of inclusions in the region from the surface to 1 / 50t (t: steel sheet thickness) in the thickness direction is preferably the same as or lower than the density of inclusions in other regions. The surface layer is particularly important for ensuring high-frequency iron loss and has a significant effect on magnetic domain movement, so a low inclusion density in the surface layer is important for reducing iron loss. In this case, a lower inclusion density in the surface layer than in other regions means that inclusions do not form smoothly on the surface, thereby significantly improving iron loss in the direction perpendicular to high-frequency rolling. The inclusion density can be measured using an electron microscope or an equivalent device (e.g., SIMS (secondary ion mass spectrometry), AFM (atomic force microscopy), EPMA (electron probe X-ray micro analyzer), etc. Both scanning and transmission electron microscopes can be used. Measurements are made for inclusions in an area of ​​1 mm x 1 mm or larger, or 1 mm x 1 mm x sample thickness (mm). The same measurement method is used for the surface and center regions, allowing for comparison of density values ​​regardless of unit. The measurement area does not include the insulating coating layer. The number of inclusions is counted for those with a diameter of at least 20 nm when converted to a spherical equivalent area method. In the case of compound inclusions, even if the shape is irregular and the parts made of components other than steel are connected, they are counted as a single inclusion and not counted further. The maximum size of inclusions is 5 μm or less. Any inclusions larger than this are excluded when counting and calculating density. However, measurements are made so that the reference area is at least 1 mm x 1 mm, even if the area occupied by large inclusions is excluded.

[0065] The non-oriented electrical steel sheet of the present invention provided as described above may have an iron loss (W15 / 1000L) of 150 W / kg or less and an iron loss (W15 / 1000C) of 150 W / kg or less. If these conditions are not met, the iron loss at high magnetic flux density will be poor, making it difficult to achieve the object of the present invention, which is to utilize the steel sheet for increasing motor output and efficiency. It is known that iron loss in the direction perpendicular to the rolling direction is closely related to the circumferential magnetic field measured at the yoke of the steel sheet. Since lower iron loss values ​​are more advantageous, the present invention does not particularly limit the lower limits of the iron loss (W15 / 1000L) and iron loss (W15 / 1000C). However, the lower limits of the iron loss (W15 / 1000L) and iron loss (W15 / 1000C) may be 50 W / kg and 50 W / kg, respectively. Meanwhile, the above W15 / 1000 means the iron loss measured by the Epstein method under the condition of a maximum magnetic flux of 1.5 T and 1000 Hz. L means the rolling direction, and C means the direction perpendicular to the rolling direction.

[0066] Furthermore, the non-oriented electrical steel sheet of the present invention may satisfy the condition 2 × (W15 / 1000C − W15 / 1000L) / (W15 / 1000C + W15 / 1000L) < 0.1. If this condition is not satisfied, it will be difficult to achieve the object of the present invention, which is to increase the magnetic anisotropy at high frequencies and utilize it to increase the output and efficiency of motors. The magnetic anisotropy at high frequencies may affect the increase in loss when the motor rotates at high speed. Since a lower value of 2 × (W15 / 1000C − W15 / 1000L) / (W15 / 1000C + W15 / 1000L) is more advantageous, the present invention does not particularly limit the lower limit of 2 × (W15 / 1000C − W15 / 1000L) / (W15 / 1000C + W15 / 1000L). However, the lower limit of the above 2×(W15 / 1000C−W15 / 1000L) / (W15 / 1000C+W15 / 1000L) may be −0.1.

[0067] The non-oriented electrical steel sheet of the present invention may have a thickness of 0.03 to 0.35 mm.

[0068] The method for producing a non-oriented electrical steel sheet according to the present invention will be described below.

[0069] First, a slab having the above-mentioned alloy composition is heated at 1050 to 1220°C. If the slab heating temperature is less than 1050°C, the temperature difference between the surface and the interior of the slab becomes large during hot rolling, resulting in poor sheet passing properties during hot rolling and an insufficient reduction ratio during hot rolling. If the slab heating temperature exceeds 1220°C, precipitates may remelt and precipitate finely after hot rolling. The lower limit of the slab heating temperature is more preferably 1080°C, and even more preferably 1100°C. The upper limit of the slab heating temperature is more preferably 1200°C, and even more preferably 1180°C.

[0070] The slab is then finish hot-rolled to obtain a hot-rolled sheet. The finish hot-rolling can be performed at 700 to 1050°C. If the finish hot-rolling temperature is less than 700°C, the shape of the hot-rolled sheet will be poor, deformation will be concentrated on the surface, making hot-rolling of the steel sheet impossible, and Goss-like orientation will increase in the surface area. If the finish hot-rolling temperature exceeds 1050°C, the friction force between the rolling roll and the sheet surface will increase, increasing the Goss-like orientation and potentially causing problems such as poor sheet shape due to high-temperature deformation. The lower limit of the finish hot-rolling temperature is more preferably 730°C, even more preferably 750°C, and most preferably 780°C. The upper limit of the finish hot-rolling temperature is more preferably 1000°C, even more preferably 960°C, and most preferably 930°C. Meanwhile, the thickness of the hot-rolled sheet may be 0.8 to 3 mm.

[0071] The hot-rolled sheet is then annealed at 850 to 1150°C for 30 to 300 seconds. If the hot-rolled sheet annealing temperature is less than 850°C, the structure may not grow or may grow finely. If the hot-rolled sheet annealing temperature exceeds 1150°C, the magnetic properties may deteriorate and the rolling workability may be impaired due to deformation of the sheet shape. The lower limit of the hot-rolled sheet annealing temperature is more preferably 900°C, and even more preferably 950°C. The upper limit of the hot-rolled sheet annealing temperature is more preferably 1135°C, and even more preferably 1110°C. If the hot-rolled sheet annealing time is less than 30 seconds, the growth of the surface and internal crystal grains differs, resulting in a disadvantage that the Goss fraction in the surface portion of the final electrical steel sheet increases significantly. If the hot-rolled sheet annealing time exceeds 300 seconds, the crystal grains become coarse, resulting in a disadvantage that the Goss fraction in the entire thickness of the finally obtained electrical steel sheet increases significantly. The lower limit of the hot-rolled sheet annealing time is more preferably 60 seconds, and even more preferably 80 seconds. The upper limit of the hot-rolled sheet annealing time is more preferably 180 seconds, and even more preferably 150 seconds. The hot-rolled sheet annealing is performed as needed to increase the orientation advantageous for magnetic properties, and can be omitted.

[0072] Thereafter, the hot-rolled sheet that has been annealed is cold-rolled to obtain a cold-rolled sheet. During the cold rolling, it is preferable that the following [Relational Expression 2] is satisfied.

[0073] [Equation 2] Maximum temperature on the surface of cold-rolled sheet during cold rolling <200 × cold rolling reduction / 100 + 40

[0074] The temperature of the steel sheet may become high due to residual heat during the hot-rolled sheet production or annealing stage, heating due to mechanical friction of the steel sheet during rolling, external heat supply, etc. If the maximum temperature of the cold-rolled sheet surface during cold rolling is 200 x cold reduction / 100 + 40 or more, a large shear force acts during material deformation, greatly increasing the fraction of crystal grains with Goss orientation in the steel sheet.

[0075] The cold rolling can be performed at a cold reduction of 35 to 98%. If the cold reduction is less than 35%, the energy stored by processing is consumed during cold rolling, and recrystallization does not occur, which is a problem in that the magnetic properties deteriorate even after annealing. If the cold reduction is more than 98%, a highly processed microstructure is formed by rolling, and there is a problem that iron loss increases in both the rolling direction and the direction perpendicular to the rolling direction even after final annealing. The lower limit of the cold reduction is more preferably 55%, even more preferably 65%, and most preferably 73%. The upper limit of the cold reduction is more preferably 93%, even more preferably 88%, and most preferably 83%. Meanwhile, the cold rolling can be a single cold rolling or two or more cold rollings with intermediate annealing in between.

[0076] During the cold rolling, the maximum rolling speed in one or more of the first and second passes may be 3 m / s or more. If the maximum rolling speed in one or more of the first and second passes is less than 3 m / s, a strong shear force acts on the steel sheet surface during rolling, resulting in increased nucleation of Goss orientation grains, which may increase the fraction of crystal grains having the Goss orientation on the steel sheet surface during final annealing. In the present invention, the faster the maximum rolling speed in one or more of the first and second passes during cold rolling, the more advantageous it is, so there is no particular upper limit. However, the upper limit of the maximum rolling speed in one or more of the first and second passes during cold rolling may be 20 m / s.

[0077] The cold-rolled sheet is then heated to the final annealing temperature. During the heating, the heating rate in the temperature range of 300 to 500°C is preferably 5 to 150°C / s. If the heating rate in the temperature range of 300 to 500°C is less than 5°C / s, recrystallization of crystal grains with orientation unfavorable for magnetic properties is promoted. If the heating rate in the temperature range of more than 300 to 500°C is more than 150°C / s, recrystallization of crystal grains with Goss orientation unfavorable for magnetic properties in the direction perpendicular to the rolling direction is greatly promoted. The lower limit of the heating rate is more preferably 7°C / s, and even more preferably 10°C / s. The upper limit of the heating rate is more preferably 120°C / s, even more preferably 100°C / s, and most preferably 50°C / s.

[0078] The heated cold-rolled sheet is then subjected to final annealing. The final annealing can be performed at 600 to 1150°C for 10 to 300 seconds. If the final annealing temperature is less than 600°C, there is a drawback that the Goss fraction inside the steel sheet increases significantly during recrystallization. If the final annealing temperature exceeds 1150°C, there is a drawback that coarse crystal grains are formed, resulting in a deterioration in high-frequency iron loss in the C direction. The lower limit of the final annealing temperature is more preferably 700°C, even more preferably 730°C, and most preferably 750°C. The upper limit of the final annealing temperature is more preferably 1120°C, even more preferably 1100°C, and most preferably 1050°C. If the final annealing time is less than 10 seconds, there is a drawback that the fraction of crystal grains having a Goss fraction in the entire sheet thickness increases significantly. If the final annealing time exceeds 300 seconds, there is a drawback that high-frequency iron loss increases significantly due to overgrowth of crystal grains. The lower limit of the final annealing time is more preferably 20 seconds, even more preferably 30 seconds, and most preferably 35 seconds, and the upper limit of the final annealing time is more preferably 240 seconds, even more preferably 180 seconds, and most preferably 150 seconds.

[0079] During the final annealing at a temperature between 500 and 750°C, the gas atmosphere preferably consists of, by volume, 15 to 99.99% hydrogen, 0.0001 to 0.0030% oxygen, and the remainder an inert gas. This control provides a substantially oxygen-free steel sheet surface. A hydrogen fraction of less than 15% results in insufficient reducing power, leading to oxidation of the surface of the material. While a hydrogen fraction of substantially 100% is ideal, industrial application of this is extremely difficult. Therefore, the hydrogen fraction is limited to 99.99%. If the oxygen fraction is less than 0.0001%, a small portion of the steel sheet surface may combine with oxygen, forming a localized oxide layer and resulting in surface irregularities. If the oxygen fraction exceeds 0.0030%, a large oxide layer may form on the surface, degrading magnetic properties and promoting the nucleation of Goss-oriented crystal grains on the surface, resulting in an increase in the surface area fraction. In the present invention, the type of the inert gas is not particularly limited, and any type used in the technical field can be used. For example, nitrogen, argon, etc. can be used.

[0080] During the final annealing, the oxygen content in the gas atmosphere in the temperature range of 800 to 1100°C is preferably 1 to 1.5 times that in the temperature range of 500 to 750°C. By controlling it in this way, there is a possibility that the growth of Goss-oriented crystal grains will not be induced. If the above conditions are not met, there is a drawback in that an oxide layer will form over a wide area on the surface, and oxides bonded to oxygen will form to a greater depth.

[0081] Meanwhile, the method may further include forming an insulating layer after the final annealing. The method of forming the insulating layer is widely known in the technical field of non-oriented electrical steel sheets, and therefore, detailed description thereof will be omitted. [Example]

[0082] The present invention will be described in more detail below with reference to examples. However, it should be noted that the following examples are intended to illustrate and explain the present invention in more detail, and are not intended to limit the scope of the present invention. The scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom.

[0083] (Example) Slabs having the alloy compositions shown in Tables 1 and 2 below were prepared, and then non-oriented electrical steel sheets were produced using the production conditions shown in Tables 3 and 4 below. Meanwhile, the remainder of the gas atmosphere in the temperature range of 500 to 750°C during final annealing was nitrogen.

[0084] The Goss orientation fraction, inclusion density, and electrical properties of the non-oriented electrical steel sheets thus produced were measured, and the results are shown in Table 5 below.

[0085] The Goss orientation fraction in the surface layer (the region from the surface to 1 / 10t (t: steel plate thickness) in the thickness direction) and the entire region in the thickness direction was measured using EBSD. To confirm the Goss orientation fraction by thickness, the RD-ND plane was observed, and EBSD measurement surfaces were separated and analyzed for the top and bottom surfaces at 1 / 10 of the total thickness. To ensure statistical reliability, the cross sections of 100 samples were measured, and the texture by thickness was measured for each sample measurement and averaged, which was evaluated as the Goss orientation fraction at 1 / 10t.

[0086] The inclusion density in the surface layer (the region from the surface to 1 / 50t (t: steel plate thickness) in the thickness direction) and other regions was measured on the surface of the steel plate using an SEM. EDS was measured at points where there was contrast in the image using the SEM, and points with element values ​​significantly higher than the Fe and steel plate composition were considered to be inclusions regardless of their size. Points with oxygen of 5% or more were considered to be oxides, points with N of 1% or more were considered to be nitrides, points with C of 1% or more were considered to be carbides, points with S of 1% or more were considered to be sulfides, etc., and the total was measured.

[0087] Iron loss (W15 / 1000L) and iron loss (W15 / 1000C) were measured using the Epstein measurement method, which is commonly used for quantitative measurements of electrical steel sheets. The samples were cut using a cutting machine, with the L-direction sample measuring 305 mm in the L direction and 30 mm in the C direction. The C-direction sample was cut so that the C direction was 305 mm and 30 mm in the L direction.

[0088] [Table 1]

[0089] [Table 2]

[0090] [Table 3]

[0091] [Table 4]

[0092] [Table 5]

[0093] As can be seen from Tables 1 to 5 above, in the case of Examples 1 to 14, which satisfy the alloy composition and manufacturing conditions of the present invention, the Goss orientation fraction and inclusion characteristics that the present invention aims to achieve are satisfied, and therefore not only are they excellent in iron loss but also in magnetic anisotropy.

[0094] Comparative Examples 1 to 10 do not satisfy the manufacturing conditions of the present invention, and do not satisfy the Goss orientation fraction or inclusion characteristics that the present invention aims to achieve. Therefore, although the iron loss is good, the magnetic anisotropy is at an inferior level.

Claims

1. In weight percent, Si: 1.5 to 6.5%, Al: 0.0005 to 3.5%, Mn: 0.01 to 3.0%, Cr: 0.005 to 5.0%, S: 0.0005 to 0.03%, and the balance being Fe and other inevitable impurities, the Goss orientation fraction in a region from the surface to 1 / 10t (t: thickness of the steel plate) in the thickness direction is 3 area % or less, A non-oriented electrical steel sheet characterized in that the density of inclusions in a region from the surface to 1 / 50t (t: thickness of the steel sheet) in the thickness direction is the same as or less than the density of inclusions in other regions.

2. The non-oriented electrical steel sheet according to claim 1, further comprising at least one of P: 0.005 to 0.08% and Sn: 0.01 to 0.2%.

3. 2. The non-oriented electrical steel sheet according to claim 1, further comprising one or more of C: 0.005% or less (except 0%), N: 0.005% or less (except 0%), O: 0.005% or less (except 0%), and Ti: 0.01% or less (except 0%).

4. 2. The non-oriented electrical steel sheet according to claim 1, further comprising one or more of Mo: 0.1% or less (except 0%), B: 0.0050% or less (except 0%), V: 0.050% or less (except 0%), Ca: 0.010% or less (except 0%), Nb: 0.0050% or less (except 0%), and Mg: 0.0050% or less (except 0%).

5. The non-oriented electrical steel sheet according to claim 1, further containing one or more of Sb: 0.1% or less (excluding 0%), Ni: 0.05% or less (excluding 0%), Cu: 0.005 to 0.2%, and Zn: 0.01% or less (excluding 0%).

6. 2. The non-oriented electrical steel sheet according to claim 1, further comprising one or more of Bi, Pb, Ge, and As in an amount of 0.20% or less (excluding 0%), either individually or in total.

7. The non-oriented electrical steel sheet according to claim 1, wherein the non-oriented electrical steel sheet satisfies the following relational expression 1: [Relationship 1] Si>Al+Mn

8. 2. The non-oriented electrical steel sheet according to claim 1, wherein the Goss orientation fraction in the entire region in the thickness direction of the non-oriented electrical steel sheet is 5 area % or less.

9. 2. The non-oriented electrical steel sheet according to claim 1, wherein the non-oriented electrical steel sheet has an iron loss (W15 / 1000L) of 150 W / Kg or less and an iron loss (W15 / 1000C) of 150 W / Kg or less.

10. The non-oriented electrical steel sheet according to claim 1, wherein the non-oriented electrical steel sheet satisfies 2×(W15 / 1000C−W15 / 1000L) / (W15 / 1000C+W15 / 1000L)<0.

1.

11. a step of heating a slab consisting of, in weight percent, 1.5 to 6.5% Si, 0.0005 to 3.5% Al, 0.01 to 3.0% Mn, 0.005 to 5.0% Cr, 0.0005 to 0.03% S, and the balance being Fe and other unavoidable impurities, at 1050 to 1220°C; finish hot rolling the slab to obtain a hot-rolled sheet; annealing the hot-rolled sheet at 850 to 1150°C for 30 to 300 seconds; cold-rolling the hot-rolled sheet after the annealing to obtain a cold-rolled sheet; heating the cold-rolled sheet; and final annealing the heated cold-rolled sheet at 600 to 1150°C for 10 to 300 seconds; During the cold rolling, the following [Relationship 2] is satisfied, During the heating, the heating rate in the temperature range of 300 to 500°C is 5 to 150°C / s, During the final annealing, the gas atmosphere in the temperature range of 500 to 750 ° C. is composed of, by volume, hydrogen: 15 to 99.99%, oxygen: 0.0001 to 0.0030%, and the balance being an inert gas; a temperature range of 800 to 1100°C during the final annealing, wherein the oxygen content in the gas atmosphere is 1 to 1.5 times higher than the oxygen content in the temperature range of 500 to 750°C. [Relationship 2] Maximum temperature of cold-rolled sheet surface during cold rolling < 200 × cold rolling reduction / 100 + 40

12. The method for manufacturing a non-oriented electrical steel sheet according to claim 11, wherein the slab further contains at least one of P: 0.005 to 0.08% and Sn: 0.01 to 0.2%.

13. 13. The method for producing a non-oriented electrical steel sheet according to claim 12, wherein the slab further contains one or more of C: 0.005% or less (except 0%), N: 0.005% or less (except 0%), O: 0.005% or less (except 0%), and Ti: 0.01% or less (except 0%).

14. 13. The method for producing a non-oriented electrical steel sheet according to claim 12, wherein the slab further contains one or more of Mo: 0.1% or less (except 0%), B: 0.0050% or less (except 0%), V: 0.050% or less (except 0%), Ca: 0.010% or less (except 0%), Nb: 0.0050% or less (except 0%), and Mg: 0.0050% or less (except 0%).

15. 13. The method for producing a non-oriented electrical steel sheet according to claim 12, wherein the slab further contains one or more of Sb: 0.1% or less (excluding 0%), Ni: 0.05% or less (excluding 0%), Cu: 0.005 to 0.2%, and Zn: 0.01% or less (excluding 0%).

16. The method for producing a non-oriented electrical steel sheet according to claim 11, wherein the slab further contains one or more of Bi, Pb, Ge, and As in an amount of 0.20% or less (excluding 0%), either individually or in total.

17. The method for producing a non-oriented electrical steel sheet according to claim 11, wherein the slab satisfies the following relational expression 1: [Relationship 1] Si>Al+Mn

18. The method for producing a non-oriented electrical steel sheet according to claim 11, wherein the finish hot rolling is performed at 700 to 1050°C.

19. The method for producing a non-oriented electrical steel sheet according to claim 11, wherein the cold rolling is performed at a cold reduction rate of 35 to 98%.

20. The method for producing a non-oriented electrical steel sheet according to claim 11, wherein the maximum rolling speed in at least one of the first pass and the second pass during the cold rolling is 3 m / s or more.

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